Apparatus for controlling electroless plating bath

ABSTRACT

A method and an apparatus for controlling the content of an electroless plating bath such as an electroless nickel plating bath such that the bath is usable for an extended period of time without remaking are disclosed. The method comprises the steps of: 
     continuously or intermittently measuring the concentration of at least one consumable ingredient of the electroless plating bath, and 
     automatically adding to the plating bath a first replenishing composition essentially consisting of the consumable ingredient after detecting that the measured value has reached a predetermined concentration; while 
     continuously or intermittently measuring one physical property of the electroless plating bath to determine the degree of aging of the bath, and 
     automatically discharging a predetermined volume of the plating solution and automatically adding to the plating bath a second replenishing composition containing unconsumable ingredients in an amount essentially corresponding to the lost amount by the discharging after detecting that the measured value has reached a predetermined value of the physical property.

This application is a divisional of copending application Ser. No.204,046, filed on Nov. 4, 1980 now U.S. Pat. No. 4,353,933.

BACKGROUND OF THE INVENTION

This invention relates to a method for controlling electroless platingbaths such as electroless nickel plating baths and an apparatus usedtherefor, and more particularly, to a method for controlling electrolessplating baths so that stable plating can be continuously conducted foran extended period of time and an apparatus used therefor.

Electroless plating baths are susceptible to severe variation of theircomposition since the metal salts, reducing agents and other ingredientsare shortly consumed during plating. It is thus necessary to frequentlycomplement such consumed ingredients to make the rate of depositionconstant and the thickness and properties of deposits uniform. For suchpurposes, a variety of methods and apparatus for the automatic controlor replenishment of electroless plating baths have been proposed asdisclosed in Japanese Patent Application Laid-Open Nos. 53-44434,53-45631 and 54-8123.

However, electroless plating baths such as electroless nickel platingbaths and electroless copper plating baths have a short effective life.As electroless plating is continued for an extended period of time,reaction by-products accumulate in the plating solution. The reactionby-products, which are believed to be decomposition products of areducing agent and neutralization salts resulting from plating reaction,adversely affect such factors as plating rate and properties ofdeposits. With reaction by-products accumulated, the replenishment ofingredients consumed by plating cannot prevent reduction of the platingrate and variation of the composition and properties of deposits.Accordingly, although the electroless plating solution is replenishedeither by the above-mentioned automatic replenishing methods or manuallywhen needed, plating baths age or become unavailable within a relativelyshort period of time. The rate of deposition is remarkably reduced andthe properties of the resulting deposits considerably vary as comparedwith those in the case of the initial plating solution, even when themetal ion and the reducing agent are replenished so as to increase theirconcentrations to the levels in the initial plating solution. Under suchcircumstances, the aged bath must be discarded and replaced by a freshplating solution.

Even in the case where the above-mentioned automatic control of anelectroless plating bath is employed to automatically replenish aplating solution, the degree of aging of the bath should always beseparately monitored to accommodate such aging. Accordingly, platingbaths must be discarded after a relatively short period of use in theprior art electroless plating processes combined with theabove-mentioned automatic control method. The prior plating processesare also unsatisfactory with respect to bath maintenance.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand an apparatus for controlling an electroless plating bath wherebystable continuous electroless plating can be carried out for an extendedperiod of time with substantial elimination of the need for remaking thenew electroless plating bath or at least with substantial retardation ofplating bath remaking as compared with the prior art method and hence,with substantial advantages in waste disposal.

Another object of the present invention is to provide a method and anapparatus for controlling an electroless plating bath whereby the rateof deposition and the properties of deposits can be kept substantiallyconstant and uniform for an extended period of time, and bathmaintenance is easy and accurate.

A further object of the present invention is to provide an electrolessplating control method and apparatus suitable in the practice ofelectroless plating of nickel, cobalt, nickel-cobalt alloy, copper, tin,gold, silver and the like.

According to one aspect of the present invention, there is provided amethod for controlling an electroless plating bath capable of using thebath for an extended period of time without remake, the methodcomprising the steps of:

continuously or intermittently measuring the concentration of at leastone consumable ingredient in the electroless plating bath, and

automatically adding to the plating bath a first replenishingcomposition essentially consisting of consumable ingredients afterdetecting that the measured value has reached a predeterminedconcentration; while

continuously or intermittently measuring one physical property of theelectroless plating bath to determine the degree of aging of the bath,and

automatically discharging a predetermined volume of the plating solutionand automatically adding to the plating bath a second replenishingcomposition containing unconsumable ingredients in an amount essentiallycorresponding to the lost amount by the discharging after detecting thatthe measured value has reached a predetermined value of the physicalproperty.

According to another aspect of the present invention, there is providedan apparatus for controlling an electroless plating bath capable ofusing an electroless plating bath for an extended period of time withoutremake, the apparatus comprising:

a mechanism for supplementing a first replenishing compositionincluding:

means for automatically measuring the concentration of at least oneconsumable ingredient in the electroless plating bath,

concentration detecting means for generating a first signal when themeasured value has reached a predetermined concentration, and

means for automatically adding the first replenishing compositionessentially consisting of consumable ingredients to the electrolessplating bath upon receipt of the first signal, and

a mechanism for discharging a predetermined volume of the electrolessplating solution and supplementing a second replenishing compositionincluding:

means for measuring one physical property of the electroless platingbath to determine the degree of aging of the bath,

aging degree detecting means for generating a second signal when themeasured value has reached a predetermined value of the physicalproperty,

means for discharging a predetermined volume of the plating solutionupon receipt of the second signal, and

means for adding to the electroless plating bath the second replenishingcomposition containing unconsumable ingredients in an amount essentiallycorresponding to a lost amount by the discharging.

In the present invention, "consumable ingredients" designate ingredientsconsumed during electroless plating, e.g. a metal ion such as nickel ionand a reducing agent such as sodium hypophosphite for nickel electrolessplating bath, and "unconsumable ingredients" designate ingredients notessentially consumed during electroless plating except by drug-out, e.g.a chelating agent such as sodium acetate, sodium citrate, etc.

In preferred embodiments of the present invention, the concentration ofthe electroless plating bath is measured in terms of the concentrationof a metal in the bath, while the degree of aging of the electrolessplating bath is obtained by measuring one physical property, that is,specific gravity, electrical conductivity or refractive index of theelectroless plating bath.

Since the concentration of an electroless plating bath, for example, themetal concentration in the bath, is measured to determine the amount ofthe consumable ingredients consumed during plating and the firstreplenishing composition essentially consisting of the consumableingredients is automatically replenished in accordance with the measuredvalue, while one physical property, for example, specific gravity,electrical conductivity or refractive index of the electroless platingbath is measured to determine the degree of aging of the bath and apredetermined volume of the plating solution is discharged and thesecond replenishing composition containing unconsumable ingredientsessentially corresponding to the discharged portion is automaticallyreplenished each time the value of the physical property measured hasreached a predetermined level (that is, each time the degree of aginghas reached a predetermined value), the bath is automatically refreshed,substantially eliminating the need for remaking a plating bath or atleast substantially extending the effective life of the bath as comparedwith the prior art method. Furthermore, since the consumable ingredientssuch as metal ions, reducing agents, etc. are maintained atsubstantially constant concentrations and the amount of reactionby-products resulting from plating process is maintained within anallowable range, the rate of deposition and the properties of theresulting deposits are maintained constant and uniform. Differentlystated, the bath is maintained under highly stable conditions for anextended period of time. It is also possible to maintain the platingbath at a lower metal concentration than the prior art bath whileretaining the rate of deposition at substantially the same level as theprior art. These features are also advantageous from a point of wastedisposal.

The present invention may advantageously be applied to electrolessplating baths of nickel, cobalt, nickel-cobalt alloy and copper.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become more apparent from the following description and claimstaken in connection with the accompanying drawings, in which:

FIG. 1 is a block diagram schematically showing one embodiment of theelectroless plating arrangement according to the present invention;

FIG. 2 is a block diagram of an example of the concentration measuringdevice;

FIG. 3 is a block diagram of a spectrophotometer section of theconcentration measuring device;

FIG. 4 is a block diagram of an example of a control section of theconcentration measuring device;

FIG. 5 is a block diagram of an example of the specific gravitymeasuring device;

FIG. 6 is a block diagram of an example of the electrical conductivitymeasuring device;

FIG. 7 is a block diagram of an example of the refractive indexmeasuring device;

FIG. 8 graphically illustrates the relationship of the number of turnsto the electrical conductivity of an electroless plating bath; and

FIG. 9 graphically illustrates the relationship of the number of turnsto the refractive index of an electroless plating bath.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, one embodiment of the arrangement for controllingan electroless nickel plating bath according to the present invention isschematically shown as including a plating tank 1 which is provided withthe necessary attachments including a heating element for heating anelectroless plating bath 2 to a desired temperature, for example, aheater or steam pipe, a cooling element for cooling the bath 2 to aapproximately room temperature after the completion of plating, afilter, a stirrer and the like, although they are not shown in thefigure.

In connection with the plating tank 1, the arrangement includes aconcentration measuring/detecting device 3 for automatically measuringthe concentration of the plating bath and transmitting signal A when themeasured value has reached a predetermined concentration level, aphysical property measuring/detecting device 4 for measuring onephysical property of the plating bath to determine the degree of agingof the plating bath and transmitting signal B when the measured valuehas reached a predetermined level (or predetermined degree of aging), afluid circulating pump 5 in the form of a microtube pump, for example,and a circulating line 6 communicating the tank 1, measuring/detectingdevices 3 and 4 and the pump 5. The plating solution 2 in the tank 1 ispumped from the inlet through the line 6 to the concentrationmeasuring/detecting device 3 where the concentration of the platingsolution 2, for example the nickel concentration, is measured and thento the physical property measuring/detecting device 4 where the selectedphysical property of the plating solution 2 is measured before it isreturned into the tank 1 from the outlet of the line 6.

FIGS. 2 to 4 illustrate an example of the concentrationmeasuring/detecting device 3. As shown in FIG. 2, the line 6 is providedwith a bypass conduit 7 which in turn, is provided with aspectrophotometer unit 8 including a flow-through cell 8a. As theplating solution 2 flows through the bypass conduit 7 and then throughthe flow-through cell 8a of the spectrophotometer unit 8, the opticaltransmittance of the solution is measured to determine the metalconcentration in the solution. In this respect, it is preferable tomeasure the absorbance of a metal complex in the plating solution inorder to follow the consumption of the metal by plating in a moresensitive manner. The transmittance of the plating solution at theabsorption wave length of the metal complex is transduced into a voltagesignal by means of a suitable transducer. The concentration of theplating solution 2 is monitored in this manner.

The concentration measuring/detecting device is described in detail byreferring to FIGS. 3 and 4. As shown in FIG. 3, light L emitted by alight source 9 in the spectrophotometer unit 8 transmits a heatradiation absorbing filter 10, a wave length-selecting filter 11 and acondensing lens 12 in this order before it transmits to the cell 8athrough which the plating solution flows continuously. The absorption oflight by the plating solution is detected by a photometer 13. Anothercell 8b containing the standard solution is mounted adjacent the cell 8aso that the cells may be alternately placed in the light path. Themeasurement cell 8a and the standard cell 8b are movable in thedirections shown by an arrow. The standard cell 8b may be moved in thelight path to replace the measurement cell 8a when needed, therebymeasuring the absorbance of the standard solution. The spectrophotometerunit 8 is electrically connected to a control section 14. As shown inFIG. 4, a minute current flow output of the photometer 13 is fed to aninput 15, amplified and converted into a voltage by an amplifier 16, andindicated by a voltmeter 17 in the form of a voltage corresponding tothe absorbance. A comparator 18 compares the output of the amplifier 16with a present voltage and produces signal A at its output 19 when theamplifier output reaches the preset value. If the preset voltage valuecorresponds to the absorbance at a given concentration to which themetal ion in the plating solution is consumed, then signal A developingat the output 19 represents that the metal concentration has reached thegiven value. Signal A is transmitted to a first replenishing mechanism20 which consists of a reservoir 21 containing a first replenishingcomposition, a supply conduit 22 connected to the reservoir 21 andopening in the plating tank 1, and an electromagnetic valve 23 in theconduit 22. Upon receipt of signal A, the valve 23 is opened for a giventime to allow a given amount of the first replenishing composition toflow into the plating tank 1 through the conduit 22. Thus the givenamount of the first replenishing composition essentially consisting ofconsumable ingredients, for example, a metal salt, a reducing agent, apH regulating agent and other consumable components is added to theelectroless plating bath corresponding to the consumption. It should benoted that although the single reservoir 21 is depicted in FIG. 1, aplurality of reservoirs may be used. Each reservoir contains a differentone of the consumable ingredients (e.g., a metal salt, a reducing agent,a pH regulating agent and other components). Of course, each reservoiris provided with its own conduit and electromagnetic valve.

By analysing the metal value, e.g. nickel value, in an electrolessnickel plating bath, in this manner, not only the amount of the metalconsumed during plating is available, but also the amount of thereducing agent consumed and the variation of pH value are available atthe same time.

In the above embodiment, the absorbance of a plating solution isdirectly measured in order to determine metal concentrations such asnickel in the plating solution. However, the method for measuring metalconcentration in the plating solution is not limited thereto, but anyother suitable means may be used. For example, an indicator or othersuitable reagents such as EDTA may be added to the plating solution tocause the plating solution to develop the corresponding color. Thedegree of color development is a measure for the metal concentration. Inthis case, however, those portions of the plating solution which havebeen subjected to concentration measurement can not be fed back to theline 6 via the bypass conduit 7 as in the case of FIG. 2. The platingsolution after measurement should be separately fed to a suitable unitfor waste disposal. Another example of concentration measurement is tomeasure the potential difference between the initial and the subsequentportions of the solution. However, the above-described method ofdirectly measuring the absorbance of the plating solution is mostadvantageous among others because it can be performed by means of simpleequipment and the plating solution subjected to measurement can be fedback to the plating tank 1 again. Furthermore, it is also possible todetect the varying concentration of a plating solution by measuring thevarying pH value thereof. Additionally, the abovementioned nickelconcentration measurement may be combined with the pH measurement sothat the pH of the plating solution may be automatically andindependently controlled in accordance with the value of the pHmeasured.

FIG. 5 illustrates one example of the physical propertymeasuring/detecting device 4. The line 6 is provided with a bypassconduit 24 with which a unit 25 is combined for measuring the specificgravity of the electroless plating solution. The specific gravitymeasuring unit 25 includes a transparent vessel 26 interposed in thebypass conduit 24 and an overflow pipe 27 such that the plating solution2 flows into the vessel 26 through the bypass conduit 24, increases itstop level, and overflows out of the vessel 26 to the downstream portionof the bypass conduit 24 through the overflow pipe 27 when the solutionlevel exceeds the inlet of the overflow pipe 27. The level of theplating solution is thus maintained constant in the vessel 26. Thevessel 26 accommodates a specific gravimeter 28 therein in the form of ahydrometer for measuring the specific gravity of the plating solution.At opposite sides of the transparent vessel 26 are located a lightsource 29 in the form of a light-emitting diode and a photoelectricdetector 30 in the form of a photo-transistor for receiving the lightemitted by the source 29. The photo detector 30 is electricallyconnected to a control section 31 wherein a minute current output of thephoto detector 30 is fed to an input 32 and amplified and converted intoa voltage by an amplifier 33. A comparator 34 compares this voltagesignal with a preset voltage value which corresponds to the voltagevalue derived from the photo detector 30 when it receives the entirequantity of light emitted by the source 29. If the input voltage is notequal to the preset voltage value, the comparator 34 produces a signal Bat its output 35.

As the plating solution 2 flows through the vessel 26, the hydrometer 28fluctuates to a more or less extent depending on the specific gravity ofthe solution. When the specific gravity of the plating solution 2 isrelatively low because of less aging and hence, the top of thehydrometer 28 does not reach the light path connecting the light source29 and the photo detector 30, the photo detector 30 receives the entirequantity of light emitted by the source 29 so that signal B is notgenerated. As the plating solution ages, the specific gravity of thesolution gradually increases and eventually exceeds the predeterminedvalue. The hydrometer 28 rises higher and the top of the hydrometer 28reaches the light path connecting the light source 29 and the photodetector 30. Since the light emitted by the source 29 is partially orentirely intercepted by the top of hydrometer 28, the photo detector 30receives none or part of the emitted light. The current valuetransmitted to the input 32 by the detector 30 is at least reduced, andthen signal B develops at the comparator output 35.

In order to measure the specific gravity of a plating solution, use mayalso be made of means for transducing the specific gravity of thesolution into a voltage, a float or scale connected to a load cell, andthe like.

FIG. 6 illustrates another example of the physical propertymeasuring/detecting device 4. The line 6 is provided with a bypassconduit 36 having a unit 37 connected therein for measuring theelectrical condutivity of the plating solution. The conductivitymeasuring unit 37 includes a cell 38 connected in the bypass conduit 36.The cell 38 consists of a substantially cylindrical tube 38a closed atboth ends and inlet and outlet members 38b and 38c connected to oppositesides of the cylindrical tube 38a. The cylindrical tube 38a is providedin the proximity of its ends with a pair of electrodes 39a and 39b,which are made of platinum or platinum plated with platinum black,spaced apart a given distance and connected to an electricalconductivity measuring instrument 41 via leads 40a and 40b. Theelectrical conductivity of the plating solution 2 in the cell 38 can bedetermined by measuring the resistance between the electrodes 39a and39b. In this case, the resistance between the electrodes 39a and 39b maybe measured as a varying current by applying a constant voltage betweenthem or as a varying voltage by flowing a constant current between them.

The above-mentioned conductivity detector 41 is electrically connectedto a control section 42 which includes an amplifier 44, a voltmeter 45,and a comparator 46. The output of the detector 41 in the form ofcurrent or voltage is transmitted to the input 43 of the amplifier 44where it is amplified and converted into a voltage when it is in theform of current. The thus converted voltage is indicated by thevoltmeter 45 as a varying voltage corresponding to the varyingconductivity and also transmitted to the comparator 46 where it iscompared with a preset voltage value. The comparator 46 produces signalB at its output 47 when the measured voltage reaches the preset value.

FIG. 7 illustrates a further example of the physical propertymeasuring/detecting device 4. The line 6 is provided with a bypassconduit 48 having a unit 49 associated therewith for measuring therefractive index of the plating solution. The unit 49 is constructedsuch that incident light L_(A) emitted by a light source 50 having acondenser combined therewith toward a prism 51 placed in the bypass line48 is totally reflected at the interface between the prism 51 and theplating solution 2, and the resulting reflected light L_(B) is receivedby a photometer 52. While the photometer 52 senses the critical light,i.e. the position and hence, intensity of which vary with the varyingrefractive index of the plating solution 2, a comparator/amplifier 53serves to actuate a servo motor 50a to move the condenser 50 to adjustor control the angle of incident light L_(A) so that the intensity ofcritical light which may otherwise vary with the varying refractiveindex of the plating solution is maintained constant. A variation of theangle of incident light L_(A) is detected in the form of a potentialdifference by an angular detector 50b.

The angular detector 50b is electrically connected to a control section54 which includes an amplifier 56, a voltmeter 57 and a comparator 58.The output of the detector 50b in the form of a voltage is transmittedto the input 55 of the amplifier where it is amplified. The amplifiedvoltage is indicated by the voltmeter 57 as a voltage variationcorresponding to the varying refractive index and also transmitted tothe comparator 58 where it is compared with a predetermined voltagevalue. The comparator 58 produces signal B at its output 59 when themeasured voltage reaches the predetermined value. The time when theincreasing refractive index of the plating solution 2 reaches apredetermined value is the time when the potential differencecorresponding to a variation of the incident light L_(A) anglecompensating for a displacement of the critical light position reaches apredetermined potential difference value.

The detection of the refractive index of the plating solution is notlimited to the above-mentioned method, but it is also possible to obtainan electrical output signal by directly sensing a variation of the totalquantity of the reflected light by means of a photometer or by directlysensing a displacement of the critical light as a variation of lightquantity.

Referring to FIG. 1 again, numeral 60 is a discharge mechanismactuatable upon receipt of the above-mentioned signal B. This dischargemechanism 60 comprises a discharge pipe 61 having one end in fluidcommunication with the plating tank 1 and a control pump 62 disposed inthe pipe. Upon receipt of signal B from the physical property detectingdevice 4, the pump 62 operates for a given time to discharge a givenvolume of the plating solution 2 out of the tank 1. Numeral 63 is amechanism for supplementing a second replenishing composition containingessential ingredients which are to be lost by the discharging. Thereplenishing mechanism 63 comprises a reservoir 64 containing the secondreplenishing composition, a feed pipe 65 having one end connected to thereservoir 64, and an electromagnetic valve 66 disposed in the pipe 65.The electromagnetic valve 66, which receives signal B from the physicalproperty detecting device 4, opens for a given time after the dischargepump 62 has been interrupted, to thereby add the second replenishingcomposition containing unconsumable ingredients in a volume essentiallycorresponding to the discharged volume into the tank 1. In this regard,the replenishing mechanism 63 may include a plurality of reservoirs eachfor different one of the ingredients to be replenished, each having anindividual feed pipe and valve.

In FIG. 1, a cooler 67 is provided in the measuring line 6 to cool theflow of the plating solution therethrough from the tank 1 to near roomtemperature. Also provided in the line 6 is a filter 68 for filteringout very fine insolubles out of the flow of the plating solutiontherethrough. The location of the cooler 67 in the line 6 upstream ofthe concentration and physical property measuring/detecting devices 3and 4 allows for measurement of concentration and physical property atroom temperature because a flow of the plating solution through thebypass conduits 7 and 24, 36 or 48 has been cooled to room temperature.For the purpose of avoiding temperature dependence of thesemeasurements, any other means may be employed as exemplified by the useof a constant-temperature bath or at temperature probe accompanied withelectrical compensation.

Described below is how to control an electroless nickel plating bathusing the above-constructed arrangement.

A workpiece to be plated is first subjected to well-known necessarypre-treatments in a conventional manner before electroless nickelplating is carried out by immersing the pre-treated workpiece in theplating bath 2 in the tank 1 heated at a desired temperature, forexample, 90° C.

In the practice of plating in this manner, the pump 5 is operated tointroduce part of the plating solution 2 from the tank 1 into themeasuring line 6. The the plating solution continuously passes throughthe concentration and physical property measuring/detecting devices 3and 4 where the concentration (nickel concentration in the case of theconcentration measuring/detecting device 3 shown in FIG. 2) and thephysical property (specific gravity, conductivity or refractive index)of the solution are automatically measured.

The progress of plating results in a reduction of the concentration ofthe plating solution, particularly the concentrations of nickel ion anda reducing agent (such as sodium hypohosphite) in the solution. In theevent a hypophosphite is used as a reducing agent for electroless nickelplating, the plating solution also shows a reduction of pH. As a resultof such concentration reduction, when the nickel concentration measuredby the spectrophotometer unit 8 reaches a predetermined concentrationvalue in the case of the concentration measuring/detecting device 3shown in FIG. 2, the control section 14 associated with the detectingdevice 3 produces signal A. Detailedly, prior to measurement of theabsorbance of the actual plating solution, the standard solution cell 8bis moved in the light path to replace the cell 8a. The absorbance ofnickel in the standard solution in the cell 8b is measured and the thusobtained absorbance value is set in the comparator 18 as a referencevalue. Then the flow-through cell 8a is moved in the light path. Whenthe nickel concentration of the solution which is given in terms of theabsorbance of the plating solution measured by the spectrophotometerunit 8 reaches the predetermined concentration value or when the voltagedetected reduces to or below the preset reference value, the comparator18 transmits signal A to the electromagnetic valve 23 of the firstreplenishing mechanism 20 to open the valve 23 for a given time.Accordingly, a given amount of the first replenishing composition is fedfrom the reservoir 21 to the tank 1 to compensate for those ingredientsconsumed during the preceding plating such as nickel ion and reducingagent and to adjust the pH of the plating bath 2. It should be notedthat the first replenishing composition mainly contains thoseingredients consumable during plating including nickel salt, reducingagent and pH regulating agent such as sodium hydroxide and optionally, aminor amount of complexing agent, stabilizer, brightener, etc. It isgenerally desired to keep the nickel salt, reducing agent and pHregulating agent separate from each other. In such separate storage,each optional agent such as complexing agent, stabilizer, and brightenermay preferably be mixed with the basic ingredient to which it is inert.

By adding the first replenishing composition in this manner, theconcentration of the plating bath 2 (the concentrations of nickel,reducing agent, etc.) is restored to the initial level so that the rateof plating or deposition is maintained substantially constant.

As reaction by-products accumulate in the plating bath 2, the values ofphysical properties such as specific gravity, conductivity andrefractive index of the plating solution vary. It is therefore possibleto determine the degree of aging of the plating bath by measuring onephysical property, for example, specific gravity, conductivity orrefractive index of the bath. The specific gravity, conductivity andrefractive index of the solution can be measured by the devices shown inFIGS. 5, 6 and 7, respectively, as described above. When the measuredvalue reaches a predetermined degree of aging (predetermined specificgravity, conductivity or refractive index value), the physical propertydetecting device 4, more specifically, the associated control section31, 42 or 54 transmits signal B to the discharge mechanism 60. Uponreceipt of signal B, the pump 62 is operated for a given time todischarge a given volume of the plating solution from the tank 1 throughthe discharge pipe 61. It should be noted that the discharge liquid isdesirably fed to a waste treatment device and essential ingredientscontained therein may be circulated for reuse after the undesiredby-products are removed. Upon interruption of the pump 62, theelectromagnetic valve 66 of the second replenishing mechanism 63 isopened for a given time to supply a given volume of the secondreplenishing composition from the reservoir 64 to the tank 1. It shouldbe noted that this second replenishing composition makes up for thoseingredients to be lost by discharging and mainly contains a complexingagent. Generally, use may be made of a solution having the samecomposition as the initially made-up plating bath or its concentrate orthose solutions mainly containing a complexing agent which is scarcelyconsumed by plating. If ingredients to be replenished such as metal saltand reducing agent tend to react with each other, it is preferred thatthey are separately stored in different reservoirs.

The discharging of a given volume of the plating solution and thereplenishing of an essentially corresponding volume of the secondreplenishing composition prevent reaction by-products from accumulatingin the plating bath to an undesired extent, thereby controlling theamount of such reaction by-products within an allowable range. As aresult, reduction of plating rate due to accumulation of reactionby-products is precluded. The plating bath is intermittently renewed inthe continued operation for an extended period of time. For an extendedoperation of the plating bath, the plating rate is maintainedsubstantially constant and deposits are maintained substantially uniformin every respect including the composition and properties, for example,Ni-P alloy composition for a hypophosphite reducing agent and thehardness of deposits. With respect to this matter, the inventors havefound that the accumulation of reaction by-products or the degree ofaging of plating bath can be accurately monitored by following thespecific gravity, conductivity or refractive index of the plating bath.When the degree of aging of the plating bath reaches a predeterminedvalue or above, a part of the plating bath is discharged and areplenishing composition mainly containing ingredients to be lost by thedischarging is replenished. As a result, the amount of reactionby-products is maintained substantially constant in the plating bath sothat the bath is effective semi-permanently or at least the effectivelife thereof is remarkably extended as compared with prior art platingmethods. For such extended use of plating baths, the plating rate,quality of deposits and other factors are maintained substantiallyuniform.

According to the above-mentioned plating process, the plating bath isautomatically controlled and maintained in an easy, but accurate manner.Especially important is the ease of maintenance of electroless platingbaths which are notoriously troublesome to handle as compared with usualelectroplating. Furthermore, since the plating bath is always maintainedat a substantially constant concentration by the replenishment, thenickel concentration of the bath may be set lower at the beginningunlike the prior art which needs to set the initial nickel concentrationto a higher level in order to compensate for a reduction of plating ordeposition rate due to reduction of nickel concentration in the progressof plating. In addition, the plating bath is automatically renewedlittle by little by means of the discharging and replenishing mechanisms60 and 63, minimizing the burden of waste treatment.

In the above-mentioned embodiments, the concentration measuring deviceis incorporated in the bypass conduit, but it may be directlyincorporated in the circulating line with omission of a bypass conduit.Also, the concentration measuring device may be provided within theplating tank.

Although the physical property measuring devices are incorporated in thebypass conduit to mesure the physical properties of the plating bathsuch as specific gravity, conductivity and refractive index in the aboveembodiments, they may be directly incorporated in the circulating linewith omission of a bypass conduit. Although the concentrationmeasurement is followed by the physical property measurement in theabove embodiments, the order of measurements is not limited thereto, butmay be reversed. Alternatively, concentration and physical property maybe measured in different independent circuits. The physical propertymeasuring device may also be provided in the plating tank.

In the above-mentioned embodiments, the second replenishing mechanism 63acts after the discharge pump 62 of the discharge mechanism 60 has beeninterrupted. Alternatively, the replenishing mechanism 63 may act by thecommand of signal B when or before the discharge pump 62 operates. Thefirst and second replenishing mechanism 20 and 63 are not limited to theabove-mentioned embodiments. For example, a dispensing pump may be usedfor the supplement of the first or second replenishing composition.

The discharge mechanism 60 may be replaced by an overflow pipe 69 asdepicted by a dot-and-dash line in FIG. 1. In this case, signal B istransmitted from the control section 14 to the second replenishingmechanism 63 to feed a given volume of the second replenishingcomposition to the plating bath in the tank 1. An incremental volume ofthe plating solution due to the addition of the second replenishingcomposition is discharged through the overflow pipe 69.

Although the above embodiments refer to the control of electrolessnickel plating, similar results may be obtained for other electrolessplating including electroless cobalt plating and electrolesscobalt-nickel plating as well as electroless coppper plating using aformalin or hypophosphite reducing agent.

Other modifications and variations may be made within the scope andspirit of the invention.

The invention will be understood more readily with reference to thefollowing examples, which are not to be construed to limit the scope ofthe invention.

EXAMPLE 1

Plating was conducted in 100 liters of an electroless nickel platingbath having the following composition and physical and chemical data:

    ______________________________________                                        Nickel sulfate (NiSO.sub.4.6H.sub.2 O)                                                              18     g/l                                              (nickel ion           4      g/l)                                             Sodium hypophosphite (NaH.sub.2 PO.sub.2.H.sub.2 O)                                                 24     g/l                                              Sodium succinate      16     g/l                                              Malic acid            18     g/l                                              Stabilizer (Pb.sup.2+)                                                                              0.003  g/l                                              pH                    5.6                                                     Specific gravity, 25° C.                                                                     1.051                                                   Electrical conductivity, 25° C.                                                              36     m.mho/cm                                         Refractive index, 25° C.                                                                     7.4    Brix %                                           ______________________________________                                    

at a temperature of 90° C. in an apparatus as shown in FIGS. 1 to 5while the nickel concentration and specific gravity of the bath wereautomatically and continuously measured (the nickel concentrationmeasurement was effected by measuring the transmittance at 670 nm). Itwas assumed that the set level of nickel ion was 4 g/l. At the time thenickel concentration measured had decreased to the set level (4%) orbelow, signal A was generated to add a first replenishing compositionconsisting of the below-mentioned three replenishing solutions I, II andIII in equal amounts of 400 ml (equivalent to 0.2 g/l of Ni²⁺) for eachaddition. These replenishing solutions I, II and III were contained inthree separate reservoirs each provided with a feed conduit and anelectromagnetic valve. In response to signal A, the valves weresimultaneously opened for a given time to feed equal amounts of therespective replenishing solutions to the plating bath. It was alsoassumed that the set specific gravity was 1.221. When the specificgravity measured had increased to the set level (1.211) or higher,signal B was generated to discharge 5 liters of the plating solutioneach time before 5 liters of a second replenishing compositionconsisting of the solution IV having the following composition was fed.(the invention method)

For the purpose of comparison, the above-mentioned plating procedure wasrepeated except that the specific gravity measurement was not effected,and hence, neither the discharge of the plating solution nor thereplenishment of the second replenishing solution IV was conducted. Thatis, only the nickel concentration was measured and the solutions I, IIand III were added accordingly. (comparative method).

After plating was conducted a given number of turns (by "one turn" ismeant the consumption of 4 g of nickel ion per liter of plating bath incontinuous plating, and hence an increased number of turns results infurther aging of the bath), the rate of deposition and the compositionof Ni-P alloy deposit were determined, obtaining the results shown inTable 1.

    ______________________________________                                        The first replenishing composition                                            Replenishing solution I                                                       Nickel sulfate         225    g/l                                             Stabilizer (Pb.sup.2+) 0.075  g/l                                             Replenishing solution II                                                      Sodium hypophosphite   270    g/l                                             Replenishing solution III                                                     Sodium hydroxide       68     g/l                                             The second replenishing solution (composition) IV:                            the same composition as the initial plating solution                          Sodium succinate       16     g/l                                             Malic acid             18     g/l                                             Nickel sulfate         18     g/l                                             Sodium hypophosphite   24     g/l                                             Stabilizer (Pb.sup.2+) 0.003  g/l                                             ______________________________________                                    

                  TABLE 1                                                         ______________________________________                                               Rate of          P content                                             Number deposition       in Ni--P deposit                                      of     (μm/hour)     (%)                                                   turns  Invention Comparative                                                                              Invention                                                                             Comparative                               ______________________________________                                        2      20.0      20.0       7.8     7.8                                       4      20.0      17.0       8.2     8.4                                       6      20.0      13.5       8.2     9.2                                       8      19.8      10.0       8.2     9.8                                       10     19.8       8.5       8.2     10.4                                      12     19.8      unplated   8.2     --                                        14     19.6                 8.2                                               16     19.6                 8.2                                               18     19.4                 8.2                                               20     19.4                 8.3                                               22     19.2                 8.3                                               24     19.0                 8.3                                               26     18.8                 8.3                                               28     18.8                 8.3                                               30     18.8                 8.3                                               32     18.6                 8.3                                               34     18.6                 8.3                                               36     18.4                 8.3                                               38     18.2                 8.4                                               40     18.0                 8.4                                               ______________________________________                                    

EXAMPLE 2

Plating was conducted in an electroless cobalt plating bath having thefollowing composition and physical and chemical data:

    ______________________________________                                        Cobalt sulfate (CoSO.sub.4.7H.sub.2 O)                                                              22.5   g/l                                              Sodium hypophosphite (NaH.sub.2 PO.sub.2.H.sub.2 O)                                                 22.5   g/l                                              Sodium tartrate       115.0  g/l                                              Boric acid            8.4    g/l                                              Thiourea              0.01   g/l                                              pH                    9.0                                                     Specific gravity, 25° C.                                                                     1.036                                                   Electrical conductivity, 25° C.                                                              26     m.mho/cm                                         Refractive index, 25° C.                                                                     5.3    Brix %                                           ______________________________________                                    

at a temperature of 90° C. in an apparatus as shown in FIGS. 1 to 5while the cobalt concentration and specific gravity of the bath wereautomatically and continuously measured. It was assumed that the setlevel of cobalt ion was 4 g/l. At the time the cobalt concentrationmeasured had decreased to the set level (4 g/l) or below, signal A wasgenerated to add a first replenishing composition consisting of thebelow-mentioned three replenishing solutions, V, VI and VII in equalamounts of 2 ml per liter of the bath for each addition. Thesereplenishing solutions V, VI and VII were contained in three separatereservoirs each provided with a feed conduit and an electromagneticvalve. In response to signal A, the valves were simultaneously openedfor a given time to feed equal amounts of the respective solutions V, VIand VII to the plating bath. It was also assumed that the set specificgravity was 1.125. At the time the specific gravity measured hadincreased to the set level (1.125) or higher, signal B was transmittedto discharge 10-50 ml/l of the plating bath each time before thecorresponding amount of a second replenishing composition consisting ofthe solution VIII having the following composition was fed. (theinvention method)

For the purpose of comparison, the above-mentioned plating procedure wasrepeated except that the specific gravity measurement was not effected,and hence, neither discharge of the plating solution nor replenishmentof the second replenishing solution VIII was conducted. That is, onlythe cobalt concentration was measured and the solutions V, VI and VIIwere added accordingly. (comparative method)

After plating was conducted a given number of turns (by "one turn" ismeant the consumption of 4 g of cobalt ion per liter of plating bath incontinuous plating), the rate of deposition and the composition of Co-Palloy deposit were determined, obtaining the results shown in Table 2.

    ______________________________________                                        The first replenishing composition                                            Replenishing solution V                                                       Cobalt sulfate          240    g/l                                            Thiourea                0.11   g/l                                            Replenishing solution VI                                                      Sodium hypophosphite    270    g/l                                            Replenishing solution VII                                                     Sodium hydroxide        68     g/l                                            The second replenishing solution VIII                                         sodium tartrate         115    g/l                                            Boric acid              8.4    g/l                                            ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                               Rate of          P content                                             Number deposition       in Co--P deposit                                      of     (μm/hour)     (% by weight)                                         turns  Invention Comparative                                                                              Invention                                                                             Comparative                               ______________________________________                                        1      14.7      14.5       4.7     4.3                                       2      14.6      14.0       4.7     4.4                                       3      14.0      13.3       4.8     4.8                                       4      14.0      12.8       4.8     5.2                                       5      13.7      11.1       --      5.7                                       6      13.4      9.1        5.0     5.8                                       7      13.0      6.3        --      6.4                                       8      12.8      unplated   5.4     --                                        9      12.7                 --                                                10     12.2                 5.4                                               11     12.1                 --                                                12     11.9                 5.5                                               13     11.5                 --                                                14     11.2                 5.8                                               15     11.0                 --                                                16     10.7                 5.9                                               ______________________________________                                    

EXAMPLE 3

Using 100 liters of the same electroless nickel plating bath as inExample 1 and an apparatus as shown in FIGS. 1 to 4 and 6, the generalprocedure of Example 1 was repeated except that the electricalconductivity of the bath was measured instead of specific gravity. Inthis example, the set conductivity was assumed to be 50 millimho (m. )per centimeter (hereinafter referred to as m.mho/cm). Signal B wastransmitted at the time the conductivity measured had increased to theset level (50 m.mho/cm) or higher.

The results obtained are shown in Table 3 together with the results of acomparative plating procedure without conductivity monitoring.

                  TABLE 3                                                         ______________________________________                                               Rate of          P content                                             Number deposition       in Ni--P deposit                                      of     (μm/hour)     (% by weight)                                         turns  Invention Comparative                                                                              Invention                                                                             Comparative                               ______________________________________                                        1      20.6      20.0       7.8     7.8                                       4      20.0      17.0       8.0     8.4                                       6      19.8      13.5       8.0     9.2                                       8      19.8      10.0       8.0     9.8                                       10     19.8       8.5       8.0     10.4                                      12     19.6      unplated   8.0     --                                        14     19.6                 8.1                                               16     19.6                 8.1                                               18     19.6                 8.1                                               20     19.4                 8.1                                               22     19.4                 8.2                                               24     19.2                 8.2                                               26     19.2                 8.2                                               28     19.2                 8.2                                               30     19.0                 8.2                                               32     19.0                 8.2                                               34     19.0                 8.2                                               36     18.8                 8.2                                               38     18.8                 8.2                                               40     18.6                 8.3                                               ______________________________________                                    

FIG. 8 graphically illustrates the relationship of the number of turnsto the electrical conductivity (m.mho/cm) of the electroless nickelplating bath having the above-described composition (measurementtemperature 25° C.). The results of FIG. 8 reveal that the conductivityincreases linearly as the number of turns and hence, the degree of agingof the electroless nickel plating bath increases. This proves that thedegree of aging of a plating bath can be accurately monitored throughmeasurement of the conductivity thereof.

EXAMPLE 4

Using the same electroless cobalt plating bath as in Example 2 and anapparatus as shown in FIGS. 1 to 4 and 6, the general procedure ofExample 2 was repeated except that the electrical conductivity of thebath was measured instead of specific gravity. In this example, the setconductivity was assumed to be 35 m.mho/cm. Signal B was transmitted atthe time the conductivity measured increased to the set level (35m.mho/cm) or higher.

The results obtained are shown in Table 4 together with the results of acomparative plating procedure without conductivity monitoring.

                  TABLE 4                                                         ______________________________________                                               Rate of          P content                                             Number deposition       in Co--P deposit                                      of     (μm/hour)     (% by weight)                                         turns  Invention Comparative                                                                              Invention                                                                             Comparative                               ______________________________________                                        1      14.6      14.5       4.4     4.3                                       2      14.4      14.0       4.5     4.4                                       3      14.4      13.3       4.7     4.8                                       4      14.0      12.8       4.8     5.2                                       5      13.8      11.1       4.9     5.7                                       6      13.6      9.1        5.1     5.8                                       7      13.2      6.3        5.1     6.4                                       8      13.2      unplated   5.2     --                                        9      13.0                 5.2                                               10     12.6                 5.4                                               11     12.4                 5.5                                               12     12.4                 5.6                                               13     12.0                 5.6                                               14     11.6                 5.7                                               15     11.4                 5.7                                               16     11.4                 5.8                                               ______________________________________                                    

EXAMPLE 5

Using 100 liters of the same electroless nickel plating bath as inExample 1 and an apparatus as shown in FIGS. 1 to 4 and 7, the generalprocedure of Example 1 was repeated except that the refractive index ofthe bath was measured instead of specific gravity. In this example, theset refractive index was assumed to be 16.0 Brix %. Signal B wastransmitted at the time the refractive index measured increased to theset level (16.0 Brix %) or higher.

The results obtained are shown in Table 5 together with the results of acomparative plating procedure without refractive index monitoring.

                  TABLE 5                                                         ______________________________________                                               Rate of          P content                                             Number deposition       in Ni--P deposit                                      of     (μm/hour)     (% by weight)                                         turns  Invention Comparative                                                                              Invention                                                                             Comparative                               ______________________________________                                        2      20.5      20.0       7.8     7.8                                       4      20.0      17.0       8.0     8.4                                       6      19.8      13.5       8.0     9.2                                       8      19.8      10.0       7.9     9.8                                       10     19.6       8.5       7.9     10.4                                      12     19.6      unplated   7.9     --                                        14     19.4                 7.9                                               16     19.4                 7.9                                               18     19.4                 7.9                                               20     19.2                 7.8                                               22     19.2                 7.8                                               24     19.0                 7.8                                               26     19.0                 7.8                                               28     18.8                 7.8                                               30     18.6                 7.7                                               32     18.4                 7.7                                               34     18.4                 7.7                                               36     18.2                 7.7                                               38     18.0                 7.6                                               40     17.6                 7.6                                               ______________________________________                                    

FIG. 9 graphically illustrates the relationship of the number of turnsto the refractive index (Brix %) of the electroless nickel plating bathhaving the abovedescribed composition (measurement temperature 25° C.).It is seen from FIG. 9 that the refractive index increases linearly asthe number of turns and hence, the degree of aging of the electrolessnickel plating bath increases. This proves that the degree of aging of aplating bath can be accurately monitored through measurement of therefractive index thereof.

EXAMPLE 6

Using the same electroless cobalt plating bath as in Example 2 and anapparatus as shown in FIGS. 1 to 4 and 7, the general procedure ofExample 2 was repeated except that the refractive index of the bath wasmeasured instead of specific gravity. In this example, the setrefractive index was assumed to be 11.5 Brix %. Signal B was transmittedat the time the refractive index measured increased to the set level(11.5 Brix %) or higher.

The results obtained are shown in Table 6 together with the results of acomparative plating procedure without refractive index monitoring.

                  TABLE 6                                                         ______________________________________                                               Rate of          P content                                             Number deposition       in Co--P deposit                                      of     (μm/hour)     (% by weight)                                         turns  Invention Comparative                                                                              Invention                                                                             Comparative                               ______________________________________                                        1      14.6      14.5       4.4     4.3                                       2      14.3      14.0       4.5     4.4                                       3      14.0      13.3       4.7     4.8                                       4      13.6      12.8       4.8     5.2                                       5      13.4      11.1       4.8     5.7                                       6      13.0      9.1        4.9     5.8                                       7      12.4      6.3        4.9     6.4                                       8      12.4      unplated   5.0     --                                        9      12.2                 5.0                                               10     11.6                 5.1                                               11     11.4                 5.2                                               12     11.0                 5.2                                               13     11.0                 5.3                                               14     10.6                 5.3                                               15     10.2                 5.5                                               16     9.6                  5.5                                               ______________________________________                                    

As seen from the results of Tables 1 to 6, the present invention isadvantageous in that the effective life of an electroless plating bathis substantially extended as compared with prior art methods.Furthermore, the rate of deposition and the physical properties ofdeposits are substantially uniform and consistent, and plating baths canbe maintained highly stable for an extended period of time sincemetallic value, reducing agent and other ingredients are maintained atsubstantially constant concentrations and the amount of reactionby-products resulting from plating process is maintained within anallowable range.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the present invention, and allsuch modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

What is claimed is:
 1. An apparatus for controlling the content of anelectroless plating bath such that the bath is capable of being used foran extended period of time without being remade, the bath consisting ofconsumable ingredients including a metal ion and a reducing agent andunconsumable ingredients including a complexing agent, the metal ionbeing reduced to a metal by the reducing agent, whereby reactionby-products including decomposition products of the reducing agent areproduced, said apparatus comprising in combination:a mechanism forsupplementing a first replenishing composition to said bath including;means for automatically measuring the concentration of at least oneconsumable ingredient in the solution of said electroless plating bath,concentration detecting means for generating a first signal when themeasured value has been reduced to a predetermined concentration, andmeans for automatically adding the first replenishing compositionconsisting essentially of consumable ingredients to said electrolessplating bath upon receipt of said first signal, and a mechanism fordischarging a predetermined volume of said electroless plating bathsolution and supplementing said bath with a second replenishingcomposition including; means for measuring the specific gravity of theelectroless plating bath to determine the degree of aging of the bath,means for detecting the degree of aging of said bath which generates asecond signal when said measured value has reached a predetermined valueof said specific gravity, means for discharging a predetermined volumeof said plating solution upon receipt of said second signal, and meansfor adding to said electroless plating bath said second replenishingcomposition containing unconsumable ingredients therein in an amountessentially corresponding to the amount discharged.
 2. The apparatus asset forth in claim 1 wherein said concentration measuring means isdesigned so as to determine the light absorbance of a metal complex insaid electroless plating bath.
 3. The apparatus of claim 1 wherein thereducing agent present in said electroless bath comprises ahypophosphite salt.
 4. The apparatus of claim 1 wherein said electrolessplating bath is selected from at least one of an electroless nickelplating bath, an electroless cobalt plating bath, an electrolessnickel-cobalt alloy plating bath and an electroless copper plating bath.